Degradation factors: The results of short sample tests for the critical current-magnetic field relationship for hard superconductors can be misleadingly optimistic if used to forecast the behavior of the wire when wound into a solenoid, unless the wire is suitably clad with a sufficient thickness of a good conductor. In the worst cases degradation of performance by factors as large as two or three or more can occur.
Protection of the coil from inadvertent quenching (returning to normal resistivity) : The energy stored in a magnetic field of 100 kOe is about 40 J/cm3; if the solenoid is of significant size (e.g., 100 cm3) and the wire becomes normal at a particular point, the magnetic energy will be dissipated there as Joule heating. The quantities involved are quite sufficient to fuse the wire and destroy the coil locally.
Energizing the coil: Even quite thin superconducting wires can carry supercurrents of many tens of amperes. Solenoids made from thicker conductors are easier to build and on inadvertent quenching the voltages built up in them will be smaller, but the introduction of large currents into a cryostat raises a number of problems.